A tiny beetle is responsible for wiping out thousands of trees in the Tijuana River Valley Regional Park, in particular willow trees.
The insect, known as the shot hole borer, is smaller than a sesame seed. It infiltrates trees with a fungus that quickly spreads under the bark.
The infestation became prevalent in the last year. One place where the devastation is visible is just off the Hollister Street bridge.
"All of those dead trees you see sticking up, those used to be green leafy trees that would block the view because they would be a wall of green willow," said Kyle Icke, Supervising Park Ranger, County of San Diego Parks and Recreation.
Icke initially thought some kind of chemical had been sprayed on the trees, until rangers discovered the shot hole borer had destroyed them.
The beetle preys upon willows because its bark is soft.
"It weakens the structure inside of the willow tree, makes it so the willow tree can't drink its water from its root and then it dies," said Icke.
California's drought has worsened the problem.
"Any natural resistance they might have is weakened, and they can't fight off the beetle as well," Icke added.
Scientists haven't found a way to stop the insect's destructive spread.
They believe the pest was somehow transported through a wood shipping crate from Southeast Asia to Southern California.
"This pest is so new to our area, they're having a hard time figuring out how to call it. We haven't figured out a way to spray or do anything like that to combat it," explained Icke. "We physically have to remove the tree and chip them up so beetles can't continue living in those trees, and continue replanting. That's the best we can do, for now."
The trees in harder-to-reach locations that aren't an impediment to park visitors, are left where they are, so the local ecosystem is not interrupted.
In some cases, park rangers let nature take its course. They have noticed some of the willows are growing back. Even though the beetles destroyed the trunks, the roots were still alive, and the trees are growing back.
"It's very hopeful to us, seeing all these willow sprouting up, they're regrowing," said Icke. "We're very happy about that."
Park rangers are also fighting the beetle's infestation in other ways.
"When we do take trees down, we're planting trees at a ratio of 3 to 1," explained Icke. "So when we lose one, we plant three more trees in its place."
They are asking visitors not to remove any wood from the parks, so the invasive pest doesn't infect even more trees.
The staff at County of San Diego Parks and Recreation is continuing to survey its parks, and is working with other agencies to see how the area fares over time.
In the interim, rangers and scientists are learning about the insect's behavior, so they can develop action plans to track and stop the species from expanding.